Problem: A circle with circumference $20\pi$ has an arc with a $\dfrac{5}{4}\pi$ radians central angle. What is the length of the arc? ${20\pi}$ ${\dfrac{5}{4}\pi}$ $\color{#DF0030}{\dfrac{25}{2}\pi}$
Solution: The ratio between the arc's central angle $\theta$ and $2 \pi$ radians is equal to the the ratio between the arc length $s$ and the circle's circumference $c$ $\dfrac{\theta}{2 \pi} = \dfrac{s}{c}$ $\dfrac{5}{4}\pi \div 2 \pi = \dfrac{s}{20\pi}$ $\dfrac{5}{8} = \dfrac{s}{20\pi}$ $\dfrac{5}{8} \times 20\pi = s$ $\dfrac{25}{2}\pi = s$